1. Field of the Invention
The present invention relates to a method for driving a display device.
2. Description of the Related Art
In recent years, a so-called self-luminous display device in which a pixel is formed using a light emitting element such as a light emitting diode (LED) has attracted attention. As a light emitting element used for such a self-luminous display device, an organic light emitting diode (also referred to as an OLED, an organic EL element, an electroluminescent (EL) element, or the like) has attracted attention, and have been used for an EL display (for example, an organic EL display or the like). A light emitting element such as an OLED is of self-luminous type; therefore, it has advantages such as higher pixel visibility, no backlight, and higher response speed compared to a liquid crystal display. The luminance of a light emitting element is controlled by the value of the current flowing therethrough.
Driving methods to control light emission gray scales of such a display device include a digital gray scale method and an analog gray scale method. In the digital gray scale method, a light emitting element is turned on/off by controlling in a digital manner to express a gray scale. On the other hand, in the analog gray scale method, there is a method of controlling the light emission intensity of a light emitting element in an analog manner and a method of controlling the light emission time of a light emitting element in an analog manner.
In the case of the digital gray scale method, there are only two states; a light emitting state and a non-light emitting state. So only two gray scale levels can be expressed. Therefore, multiple gray scale display is attempted by using another method in combination with the digital gray scale method. As the method for achieving multiple gray scale, a time gray scale method is often used (see Reference 1: Japanese Patent Laid-Open No. 2001-324958 and Reference 2: Japanese Patent Laid-Open No. 2001-343933).
Displays in which a display mode is controlled in a digital manner and a time gray scale method is combined to express a gray scale, include displays other than an organic EL display using a digital gray scale method. An example thereof is a plasma display.
The time gray scale method is a method for expressing a gray scale by controlling the length of a light emitting period or the number of light emissions. In other words, one frame is divided into a plurality of subframes, each of which is weighted by the number of light emissions, a light emitting period, or the like, and the total weight (the sum of the number of light emissions or the sum of the light emitting periods) is differentiated for each gray scale level, thereby expressing a gray scale.
Thus, there are an analog gray scale method and a digital gray scale method; however, both methods have advantages and disadvantages, and there has been no method that combines the advantages of both methods. Therefore, the method has to be limited to either of them.
For example, in the case of an analog gray scale method, a gray scale is displayed smoothly, whereas noise is also displayed together or contrast is decreased.